The group's name stems in part from astrology in that all four members are "fire signs": Austin is an Aries, Proctor's sign is Leo, Bergman and Ossman are both Saggitarius.

The group's name also references an early television series called Fireside Theatre, which ran on NBC from 1949 to 1958, as well as to the Fireside Chats radio broadcasts of President Franklin Delano Roosevelt (a parody of which can be heard in one of the Theatre's Nick Danger adventures.

The Firesign Theatre employs a stream of consciousness style that includes direct references to movies, radio, TV, political figures, and other cultural sources, intermingled with sound effects and bits of music. The resulting stories - including the theft of a high school, a fair of clowns and holograms and aliens who use hemp smoking to turn people into crows - border on psychedelia, an effect intensified by the frequent appearance of mock "advertisements" satirizing real products.

While their stream of consciousness style has the feel of improvisational comedy, most of the material is actually tightly scripted and memorized. The group's writing method demands the consent of all four members before a line can be included. Much of their work has been copyrighted under the name "4 or 5 Crazee Guys."